Controlled oscillator



June 5, 1956 P. G. HANsl-:L

CONTROLLED OSCILLATOR Original Filed Dec. 9, 1950 United States Patent CONTROLLED OSCILLATOR Paul G. Hansel, Greenvale, N. Y., assignor to Servo Corporation of America, New Hyde Park, N. Y., a corporation of New York Original application December 9, 1950, Serial No. 200,060. Divided and this application April 23, 1952, Serial No. 283,922

6 Claims. (Cl. Z50- 36) My invention relates to a secondary frequency standard, as for frequency-metering applications. This application is a division of my copending application Serial No. 200,060, filed December 9, 1950.

It is an object of the invention to provide an improved device of the character indicated.

It is another object to provide an improved directreading frequency meter adapted for the measurement of unknown frequencies and for the comparison of frequencies.

It is also an object to provide an improved referencefrequency generator to deliver a known frequency as a secondary standard.

Another object is to provide an improved means for therprovision of error signals, representing frequency deviations of a given signal from a reference signal (of known frequency), as for application to the stabilized frequency control of the first-mentioned signal.

It is a still further object to provide a secondary frequency standard or meter requiring no point-by-point calibration procedures.

It is a general object to meet the above objects with a frequency meter capable of indicating directly, to any arbitrarily desired resolution, the frequency of an injected signal, the operation being entirely automatic and the equipment relatively simple.

Other objects and various further features of the invention will become apparent or will occur to those skilled in the art from a reading of the following specication in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, a preferred form of the invention- Fig. 1 is a schematic layout of a frequency meter incorporating features of the invention;

Fig. 2 graphically depicts a frequency-'scanning and counting operation in the device of Fig. l; and

Fig. 3 schematically illustrates one form of the spectrum generator in Fig. 1.

Briefly stated, my invention contemplates establishment of a spectrum of harmonic frequencies to be used as reference frequencies. This spectrum may be based upon a single crystal oscillator so that all harmonics may reilect the accuracy of the single crystal; the spectrum may include harmonics of a submultiple of the base frequency of the crystal. Frequency-responsive spectrum-probing means, which may include a free-running search oscillator, may be tuned continuously over a frequency range including a band within the said harmonic spectrum, and a counter may respond to frequency coincidences between the tuned condition of the probing means and the spectrum of harmonic frequencies, so as to count the number of such coincidences for a particular `tuning search or sweep of the probing means. In use as a frequency meter, that is, when it is desired to measure an unknown frequency, the probing means may be caused panoramically to scan a given part of the harmonic spectrum, with the counter responding to the number of harmonics traversed by the probing sweep. When the oscillator reaches the unknown frequency, the counter responds. to the latter coincidence to stop further counting, and the final count is preserved as a frequency indication.

To simplify the counting procedure, the invention additionally provides a narrow band-pass filter capable of selecting a particular one of the harmonics in the harmonic spectrum, andthe coincidence between the tuned probe condition and the lter-selected frequency may be utilized to derive a reference signal to start operation of the counter. Thus, the counter need only respond to the number of harmonics traversed in the total sweep, beginning at the filter-selected harmonic and extending to the harmonic immediately preceding the unknown frequency.

In use as a secondary frequency standard, manual means may be provided in conjunction with the counter to select a particular frequency, and the tunable probing means may be cause to sweep over the spectrum until the counter has attained the count which has been pre-selected as representing the desired signal frequency. Upon attainment of this count, an automatic frequency-control circuit may be activated so as to stop further sweep of the probing means and to stabilize the oscillator at the thenattained particular harmonic frequency. In use as an error-signal generator, as for slaving a transmitter oscillator to a given selected secondary-standard source frequency, the transmitter-oscillator output frequency may bemixed with the above-mentioned preselected secondarystandard source frequency, and known means may respond to the resulting beat frequency to derive an error or deviation signal.

Referring to the drawings, a harmonic spectrum may be generated by a crystal-controlled harmonic-spectrum generator l0 delivering a spectrum of signals. Each signal may be of a different frequency, but having an integral harmonic relationship to the basic crystal frequency F0. A suitable technique to provide such a spectrum is illustrated schematically in Fig. 3; according to this technique, and depending upon the desired resolution of the device, a particular submultiple of the natural crystal frequency Fo is selected, and the harmonics of the resulting submultiple frequency fo are employed. As a means for arbitrarily selectingv a base frequency from which the counting function is to operate, I provide a single-frequency selection lter I1, which may be a narrow bandpass lter having a pass band that is preferably no greater than the frequency spacing between adjacent harmonics in the spectrum of generator 10. The filter 11 may be of a manually tuned variety, and the setting of the control knob 12 may determine various levels (as, for eX- ample, given multiples of the basic frequency F0 of the crystal) of single-frequency selection within fthe total spectrum of generator 10, depending upon the general region within the spectrum in which the unknown frequency fu lies or is thought to lie. The output of the lter 11 may thus be a selected reference frequency fr.

In order panoramically to scan or sweep the spectrum of generator 10, I employ frequency-responsive spectrumprobing means which may include a tuned circuit with means for sweeping the tuned frequency thereof throughout the harmonic spectrum and with detector means responsive to frequency coincidences in the harmonic spectrum; however, in the present form, I provide a free-running oscillator 13, which may employ conventional sweep means 14, such as a motor drive on the oscillator tuning capacitor, or a reactance tube controlled by a sweep circuit. The output of oscillator 13 may be combined in a non-linear element or mixer 15 to derive a signal upon tion 16 to a counter-control circuit 17 for the digital counter 18. The counter 18 may include a display window 19 within which will appear a given number representing the solution of the counting process.

Once the counter-control circuit has been enabled by a start-count impulse delivered in line 16, the countercontrol circuit 17 may respond to all register impulses delivered in line 20. It will be understood that such register impulses may be derived in a manner analogous to derivation of the start-count impulse, as by connecting a mixer 21 to respond both to the sweep frequency fs of the oscillator 13 and to the full harmonic spectrum (fo ufo) of generator 10.

The unknown or undetermined frequency fu may be applied to my frequency meter at a terminal 22 for direct connection to one side of a mixer 23, which is also responsive to the sweep frequency fs produced in the output of oscillator 13. Upon coincidence of the sweep frequency fs with the frequency fu of the unknown signal,

the mixer 23 may generate a control impulse for stopping the count or for reading the count registered by the counter 18, as will be understood.

In Fig. 2, I have displayed a portion of the spectrum in which the unknown frequency fu occurs. In the case shown, the unknown frequency happens to occur at a point which fails to coincide with one of the harmonic frequencies in the spectrum produced by generator 10. In order to determine a reference frequency or harmonic near the unknown frequency fu, I adjust the position of the knob 12 so that a reference frequency fr occurs in the general neighborhood of the unknown frequency; in the case shown, the reference frequency selected is 100 mc. In the normal automatic tuning cycle of the sweeposcillator means 13-14, the entire (or a relatively large part of the) harmonic spectrum will be swept, but no count will be registered until the counter-control circuit 17 is enabled, upon coincidence of the sweep frequency fs with the reference frequency fr. Thereafter, the counter-control circuit 17 may respond to all the register impulses produced upon sweeping the oscillator frequency fs past successive harmonic frequencies in the spectrum of generator 10. In the case shown in the drawings, the spaced frequency marks in the spectrum will be understood to be purely diagrammatic, in that too few are depicted on the present scale to provide any great number of significant figures in the count indication at window 19; but it will be understood, as suggested above, that the harmonic marks in the spectrum may be as close together as desired, depending upon the desired resolution of the system.

Upon coincidence of the sweep frequency fs with the unknown frequency fu, an impulse will be generated by mixer 23 to stop further counting. The counter 18 will then display an indication of the difference between the single frequency selected by'placement of knob 12 and the most recently swept harmonic frequency, that is, the harmonic swept just prior to coincidence with the unknown frequency fu. If desired, interpretation of the counter reading may be facilitated by means of a mechanical connection, designated schematically by the dotted line 25, between the knob 12 and the counter-control circuit 17, so that all counts following an enabling of the counter-control circuit 17 may be added to a counterdisplay number representing the single reference frequency fr selected by the filter 11; the display at window 19 may then be direct-reading.

It will be understood that if it is desired to resolve the unknown frequency with finer detail, conventional methods of interpolation may be employed to determine the spectral placement of the unknown frequency between adjacent harmonic frequencies in the spectrum of generator 10.

As indicated generally above, my equipment may be further utilized for the generation of a selected secondary standard frequency, based on the harmonic output of the generator 10. In such applications, a fixed-count selector knob 26 may be employed in conjunction with the counter-control circuit 17 to select a given counting limit at which an enabling signal may be produced in an output line Z7. Further, switches 28-29 (shown by a dashed-line interconnection to be mechanically ganged) may be closed to introduce an automatic-frequency-control circuit 30 into controlling relation with the oscillator 13, once activated by an enabling signal in line 27. If desired, a sweep-stopping relay or other means 31 may also respond to activation of the automatic-frequencycontrol circuit 30 for the positive stopping of further sweep tuning, so that the oscillator 13 may be more readily stabilized by the automatic-frequency-control circuit 30.

In operation as a secondary-frequency-standard source, the knob 26 may be turned until the desired source frequency is read on the counter display 19. The sweeptuning means may then be activated so as to cause sweeping through the spectrum of generator 10. Upon passing the frequency selected by filter 11, the start-count impulse in line 16 will enable the counter-control circuit, and counting will proceed with each successive register impulse in line 20 until such time as the count selected by the knob 26 has been reached. Upon reaching the selected count, an impulse from circuit 17 will be delivered in line 27 to activate the automatic-frequencycontrol circuit 30, which may then respond to the particular harmonic frequency which caused such response. Activation of circuit 30 serves to stop further sweep tuning of the oscillator 13 and to slave the oscillator 13 to that particular harmonic frequency. The output of oscillator 13 may then be available at terminal 32 to provide the desired output signal.

As also indicated generally above, my equipment may be utilized as a means for stabilizing an external oscillatory circuit which it may be desired to tune to a selected secondary standard frequency. In such case, the output frequency of the external circuit may be applied as the unknown signal fu at terminal 22, and, once the oscillator 13 has been slaved to the automatic-frequency-control circuit 30, the mixer 23 may yield an error signal of frequency fe representing the instantaneous deviation of the externally generated frequency fu from the secondarystandard frequency then available as the oscillator frequency fs. If desired, known means (not shown) may respond to the error-signal frequency fe to derive an error signal of magnitude proportional to frequency deviation from the standard frequency.

It will be appreciated that I have described a relatively simple frequency-standard control means having direct application to the metering of frequencies and to the generation of known frequencies for external comparison. My equipment may be based entirely upon a single crystal oscillator, and no point-by-point calibration procedures are required, as in present-day frequency meters.

While the invention has been described in considerable detail and a preferred form illustrated, it is to be understood that various changes and modifications may be made within the scope of the invention as defined in the appended claims.

I claim:

l. A secondary-standard-frequency source, comprising a reference oscillator having a known base frequency, frequency-dividing means connected to said oscillator for deriving a submultiple of said base frequency, harmonicgenerating means controlled by said dividing means for deriving a spectrum rich in harmonics of said subrnultiple frequency, a single-frequency selection filter connected to said generating means for passing a single one of said harmonics, a second oscillator tunable over a range of frequencies including said one harmonic, counter means connected to said filter and to said second oscillator and to said generating means and responsive to frequency coincidences between the second-oscillator output and said one harmonic and other harmonics in said spectrum, said counter means including selector means for selecting a given count of said counter` means, said given count being identified with a desired secondary-standard frequency to be generated, and automatic-frequency-control means connected to said counter means and to said second oscillator and effective upon attainment of said given count to hold said second oscillator tuned to that spectrum harmonic which was responsible for producing said given count,

2. A secondary-standard-frequency source, comprising a reference oscillator having a known base frequency, frequency-dividing means connected to said oscillator for deriving a submultiple of said base frequency, harmonicgenerating means connected to said dividing means for deriving a spectrum rich in harmonics of said subrnultiple frequency, harmonic-selection means including a tunable single-frequency selection lter connected to said generating means for passing a single selected one of said harmonies for any one of several selectively tuned conditions thereof, a second oscillator tunable over a range of frequencies including the selected harmonic frequency, said second oscillator including sweep means for automatically tuning said second oscillator, counter means connected to said iilter and to said second oscillator and to said generating means and responsive to frequency coincidences between the instantaneous second-oscillator output and said selected harmonic and other harmonics in said spectrum, said counter means including selector means for selecting a given count of said counter means, said given count being identified with a desired secondarystandard frequency to be generated, and sweep-stopping means connected to said counter means and to said second oscillator and responsive to said counter means to effectively hold the instantaneously developed swept frequency upon attainment of said given count, whereby said second oscillator may remain tuned to the frequency which coincides with the spectrum harmonic responsible for producing said given count.

3. A device according to claim 2, and including means connected to said second oscillator and including an extermal-frequency supply connection and responsive to the frequency difference between an externally supplied frequency and the coincidence frequency representing said given count, for generating a frequency-deviation signal.

4. In a device of the character indicated, a crystalcontrolled harmonic-spectrum generator, a single-frequency selection iilter connected to said generator and passing one of the harmonic frequencies of said generator as a reference frequency, f iid filter including selector means for selecting the particular one of said harmonics to which a count is to be referenced, a wide-range oscillator tunable over a relatively wide portion of the spectrurn of said generator including the selected harmonic, iirst mixer means connected to said filter and to said oscillator, said iirst mixer means being responsive to said oscillator and to said reference frequency for deriving a count-control impulse upon an input frequency coincidence, second mixer means connected to said generator and to said oscillator, said second mixer means being responsive to said oscillator and to the spectrum of said generator for deriving a register impulse upon each input frequency coincidence, a counter and countercontrol means connected to both said mixer means and responsive to impulses from both said mixer means, and means interlocking said selector means and said countercontrol means, whereby said counter may be direct reading Whatever the reference harmonic selected by said selector means.

5. A device according to claim 4, in which third mixer means is connected for response to said oscillator and to an externally supplied signal for deriving a further countcontrol impulse upon an input frequency coincidence, and in which said counter-control means is connected to and responds additionally to said third mixer means.

6. A device according to claim 4, in which said countercontrol means includes a iixed-count selector, and in which automatic-frequency-control means connected to said counter-control means is connected in controlling relation with said oscillator upon attainment by said counter-control means of the particular response that is determined by said fixed-count selector.

References Cited in the iile of this patent UNITED STATES PATENTS 2,451,320 Clammer et al. Oct. 12, 1945 2,452,960 Smith Nov. 2, 1948 2,476,840 Colander July 19, 1949 2,523,106 Fairbairn Sept. 19, 1950 2,539,673 Peterson Ian. 30, 1951 FOREIGN PATENTS 122,482 Australia Oct. 12, 1948 

